1. Field of the Invention
The present invention is related to the field of servicing wellbores with electric wireline tools. More specifically, the present invention is related to the use of electric wireline tools which are conveyed into a wellbore by using coiled tubing. Coiled tubing is typically used to transport servicing tools, including electric wireline tools, into wellbores which are directionally drilled to high inclinations.
2. Description of the Related Art
Coiled tubing conveyance of servicing tools, including electric wireline tools, into wellbores, is known in the art. For example, "World Oil's Coiled Tubing Handbook", Gulf Publishing Co., Houston, Tex., 1993, describes a number of the applications of coiled tubing for servicing wellbores.
To convey electric wireline tools into a wellbore with coiled tubing, it is first necessary to insert an electric wireline, or cable, coaxially into the coiled tubing. After insertion of the cable into the coiled tubing, one end of the cable is connected to a cable head, which makes electrical and mechanical connections from the cable to the electric wireline tools. The cable head is affixed to the coiled tubing in such a way that the motion of the coiled tubing as it traverses the wellbore is transmitted to the electric wireline tools, or tool string, directly through the cable head and not through the cable itself.
A pump which circulates fluid from the earth's surface, through the coiled tubing and into the wellbore is typically required for servicing a wellbore with coiled tubing. Circulation of fluid into the wellbore can be used for such purposes as cleaning the wellbore of drilling debris, and maintaining control of fluid pressure in the wellbore particularly in cases where the wellbore may penetrate an earth formation with a fluid pressure which exceeds the hydrostatic pressure of the fluid in the wellbore. Maintaining control of fluid pressure in the wellbore requires the ability to seal the wellbore in the event of pump failure or a leak in the coiled tubing which would make using the pump impossible.
Safety considerations require that the annular space between the exterior of the coiled tubing and the interior of the wellbore be sealable at or near the earth's surface to prevent uncontrolled escape of fluid from the wellbore through the annular space. Safety considerations also require that the coiled tubing be hydraulically sealed. Uncontrolled fluid escape from the wellbore, known as a "blowout", can be caused by penetrating an earth formation with a fluid pressure which exceeds the hydrostatic pressure of fluid in the wellbore if the annular space and the coiled tubing are not hydraulically sealed. An annular seal is typically attached to a casing flange at the top of the wellbore for sealing annular space to prevent a blowout.
The hydraulic seal in the coiled tubing can be lost if the tubing develops a leak above the annular seal. In the case of a leak in the tubing above the annular seal, equipment attached to the top of the wellbore, which typically includes a set of hydraulically operated rams, can completely close the wellbore. Closure is achieved by severing any device passing through the rams, which can include the coiled tubing. After severing the device, the rams themselves close to seal the wellbore.
Using the rams with coiled tubing in the wellbore is not desirable because the coiled tubing would be severed if the rams were closed. Severed coiled tubing is very difficult to recover from the wellbore, since a drilling rig, or similar system with vertical lifting capability, will typically not be in use at the wellbore when the wellbore is being serviced with coiled tubing.
It is known in the art to use a one way flow or "check" valve to supplement the rams and the annular seal for preventing uncontrolled fluid discharge from the wellbore. The check valve is typically attached to the end of the coiled tubing which is inserted in the wellbore. The check valve prevents fluid entry into the coiled tubing from the wellbore. In the event the wellbore penetrates a formation with a fluid pressure which exceeds the hydrostatic pressure of the fluid in the wellbore, the check valve could prevent a blowout, particularly if the coiled tubing were to develop a leak above the annular seal. Because the fluid in the wellbore is prevented from entering the coiled tubing by the check valve, the annular seal alone can usually seal the wellbore in the event of a leak in the coiled tubing located above the annular seal.
It is also known in the art to use a check valve in combination with a cable head adapted to be affixed to the end of the coiled tubing. The check valve known in the art typically comprises a spring loaded ball and a substantially conically shaped valve seat machined into the cable head. The ball is pushed into the valve seat by the spring. The check valve known in the art has problems during repeated operation. Fluids pumped through the coiled tubing can contain materials which deposit on the valve seat and cause the check valve to lose sealing capability.
It is an object of the present invention to provide a wireline cable head, adapted to be affixed to one end of a coiled tubing, the cable head having a check valve that is resistant to seal loss.